This invention relates to an apparatus and a method for heat sealing overlapping ends of thermoplastic strap.
In the packaging field, it is common practice to provide tensioned bands (usually in the form of steel straps) about single packages or groups of packages for reinforcement and for securement purposes. The steel bands or straps usually are held in place by sleeve-like seals which are crimped about overlapping ends thereof. In some cases, the overlapping ends are spot welded together. Recently, as a substitute for steel band strapping material, straps formed of various polymeric materials have been utilized. These particular strapping materials are thermoplastic and usually are secured by heat sealing overlapping strap ends to one another.
In U.S. Pat. No. 3,368,323 to Wood there is disclosed a method which comprises positioning the end portions of the strap in face-to-face spaced apart relationship; inserting a heating element between the end portions of the strap and pressing the end portions of the strap against the heating element while it is hot to cause the strap surfaces engaging the heating element to soften; removing the heating element by sliding in a longitudinal direction of the strap from between the end portions of the strap; and pressing the softened surfaces of the strap together. The heating element is described as being "preferably in the form of a flat blade".
In a commercial apparatus similar to that of the Wood patent, the hot knife blade is flat, with a thickness of about one-eighth of an inch, and has tapered blade ends that are ground to a point. A major drawback encountered in the use of this apparatus has been that it has not been possible to consistently produce welds having a high joint strength of at least 70 percent.
U.S. Pat. No. 3,316,687 to Bartlett also discloses a method for securing ends of thermoplastic strap by hot knife techniques. In this method, a thin blade heating element is positioned between two spaced apart ends of thermoplastic strap to soften the opposing face areas of the strap ends. Preferably, the heating element does not contact the opposing face areas, but is sufficiently hot so that the opposing face areas soften by radiant heat transfer in a short period of time. Contact with the heating element is preferably avoided in order to prevent a coating of strap material from building up on the heating element. Once the face areas are softened, the heating element is withdrawn, and the strap ends are clamped together and held under pressure to effect a weld.